I may be over thinking this portrait but to me there is so much symbolism:
-Katniss not sitting in the chair, because she doesn’t abide by the Capitols rules
-She is the only one so far looking directly at us, to show that she isn’t intimidated by the Capitol, but the others were.
-She’s holding the white rose down to show her feelings of superiority over it, with President Snow being the rose.
-Unlike the others, Katniss has a simple and plain chair. It’s because Katniss was a simple girl from The Seam who became the symbol of the rebellion.